1. Field of the Invention
This invention is related to printed circuit boards, and more particularly to an improved method for forming multilayer printed circuit boards.
2. Prior Art
Multilayer printed circuit boards are well known in the art and have advanced to the point where they are becoming more and more capable of supporting high density circuits, modules, etc. Often, in providing the increased density capability for supporting circuitization, it has become necessary to increase the number of signal, ground and voltage planes found in a multilayer printed circuit board, as well as the conductor density in each signal plane. In some applications, in order to minimize the number of interboard connections, it has become desirable to utilize as large and as dense a multilayered printed circuit board as possible. However, as the size of the printed circuit board is increased, it becomes more and more difficult to maintain registration between the various interplanes, because the various interplanes may have different expansion characteristics, especially during the processing steps of forming the printed circuit board. Obviously, the registration problem increases as the number and density of interplanes in the final product is increased.
In the process of forming a printed circuit board, it has been a standard practice in the industry to begin with an insulative base material to circuitize one or both surfaces of the base material and to combine a number of these to form a multilayer board. Typical of the prior art are the processes shown and described in U.S. Pat. Nos. 3,554,877; 3,791,858 and 3,867,759. In U.S. Pat. No. 3,554,877, individual double sided printed circuit boards are laminated together to form a multilayer printed circuit board. Similarly in U.S. Pat. No. 3,867,759, a plurality of strip transmission line printed circuit board segments are used to form a plurality of printed circuit boards with each board to be a layer in and of a multilayer integrated package. In U.S. Pat. No. 3,791,858, a multilayer printed circuit board is produced using additive techniques for forming conductors within each layer of the multilayer board, with the initial layer being built up on an insulative base material, such as of glass cloth-epoxy resin.
Generally, in the process of forming the circuitized assemblies, it is necessary to provide a pluralityof interconnections, commonly called "vias", by drilling holes in the base material and plating interconnections in the "vias" when the surfaces of the base material are circuitized. If the "vias" are not properly aligned with electrical interconnection pads at the end of signal lines forming the circuit patterns, the proper electrical interconnections between the planes cannot be established. Obviously, as the number and density of planes is increased with interconnections therethrough, it becomes especially important that relative absolute registration be maintained.
Inherent in the problems of the prior art devices is that the base building block of the insulative material is somewhat unstable as it is processed through varying temperatures and pressures and subjected to various chemical treatments. As a result, there can be significant shifts in the relative positioning of the circuitized lines built up on the intermediate packages, which provide registration problems in the assembly of a final package. This problem is, as mentioned above, significantly increased as the dimensions and densities of the printed circuit board are increased.
An additional problem that has occurred with prior art processes is that the dielectric base material in addition to being dimensionally unstable is often thin, flexible and fragile. It was not uncommon when plating thin line circuits on such base material to find that the circuit lines would break or rupture as the material was handled, which problem also becomes more significant as the size of the printed circuit board is increased.